This application claims the priority of European patent application No. 00 120 616.8, filed Sep. 21, 2000, the disclosure of which is incorporated herein by reference in its entirety.
When machining a workpiece such as for instance a crankshaft with very high precision it is sometimes desirable to perform this machining (turning, grinding etc.) in part between centers and in part with a workpiece that is clamped in the jaws of the chuck of a machine-tool. This is because in order to machine parts of the workpiece that are far away from its extremities and possibly eccentric, this workpiece must be firmly clamped at least at one of its extremities in the jaws of a chuck, which means that this machining cannot be done between centers. Now, in order to obtain a precise centering of the workpiece when it is clamped in the chuck its jaws must size a surface that has a previously been machined with a high precision with respect to a given rotation axis. It is, however, difficult, or even impossible, to perform the previous machining of this surface (which can for instance be the end bearing of a crankshaft) while corresponding extremity of the workpiece is clamped in a chuck, either because the surface to be machined would then be so near to the chuck that it could not be reached by tools, or because if said surface remains readily accessible there is no place near that end of the workpiece where the chuck could size it. Therefore, said end is often machined in a first, separate operation, between rotating center pins, which ensures a very precisely centered surface. This in turn guarantees that the workpiece will be precisely centered for later operations when it is seized by jaws that grip said previously machined surface, for example in order to perform an eccentric grinding of the workpiece.
In order to work on a workpiece that is sometimes maintained between a center pins and sometimes in the jaws of chuck of the machine-tool it is known to use a center pin co-axial with the chuck and displaceable along its rotation axis so that in its foremost position it can be gripped by the jaws of the chuck. However, because of the required precision and the necessity to move the pin longitudinally, it was hitherto not possible to journal it satisfactorily.
Hence, it is general object of the invention to support the pin in a way that allows one to move it axially when this is desired, but also to easily immobilize it in a position where it is centered with high degree of precision. In order to implement this and still further objects of the invention, which will become more readily apparent as the description proceeds, the invention is defined as recited in the claims.